This invention relates to the removal of the clear water layer which forms between a floating scum, and descending sludge layer in sequential batch reactors (SBR's), and similar devices. An SBR is a large tank with mixing and aerating means; it is filled with raw sewage during the fill cycle. The sewage is then vigorously aerated and mixed with appropriate chemicals during the aeration cycle (approximately 8 hours). After the aeration cycle, the tank is allowed to go into a quiescent period, a scum layer forms at the surface, and the heavier sludge begins to settle to the lower part of the tank. The clear liquid which is present between the scum and sludge is referred to as "supernatant", and is suitable for discharge. Without a decanter, it can take as much as 3 hours for the sludge to settle, and another 3 hours to decant the supernatant. This invention provides a means to exclude the raw sewage, sludge, and scum from entering the decanting mechanism, and yet provides a method of decanting the supernatant at a constant rate, thus removing uncontaminated supernatant as the sludge settles saving as much as 3 hours per SBR cycle.